Heart of Darkness is a title with multiple layers. On the most superficial layer, it refers to the continent of Africa where the story (within the story) takes place. Marlow speaks of the how, as he motored the steamboat into the continent, "the reaches opened before us and closed behind, as if the forest had stepped leisurely across the water to bar the way for our return" as they "penetrated deeper and deeper into the heart of darkness." "The steamer," he says, "toiled along slowly on the edge of a black and incomprehensible frenzy." The continent itself is forbidding to a "civilized" man, being thick with impenetrable foliage, of "massive, immense trees" that close onto the river.
Another type of "darkness" they are penetrating is the natives. He has cannibals working on the boat with him, which he says are "fine fellows...in their place." As they chugged along--and their pace seemed sluggish indeed--they would encounter "a burst of yells, a whirl of black limbs, a mass of hands clapping, of feet stamping, of bodies swaying, of eyes rolling, under the droop of heavy and motionless foliage. The steamer toiled along slowly on the edge of a black and incomprehensible frenzy." It seemed like they were among "prehistoric man," or in a "madhouse." The cultures and customs and language were all foreign to Marlow, and he reacted with the characteristic anxiety of a "civilized" man. To him, the Africans he worked with and saw we not only dark in flesh, but dark in custom, culture, and spirit.
If we peel away the next layer of the onion, we begin to see that the "heart of darkness" Conrad is really exposing is that of the "civilized" Company men who live among the natives, enslave them, and abuse them. At one of his first stops upriver, he docks and walks toward a shade to cool off and finds, much to his horror:
black shapes crouched, lay, sat between the trees, leaning against the trunks, clinging to the earth, half coming out, half effaced within the dim light, in all the attitudes of pain, abandonment, and despair. ...This was the place where some of the helpers had withdrawn to die.
They were dying slowly—it was very clear. They were not enemies, they were not criminals, they were nothing earthly now,—nothing but black shadows of disease and starvation, lying confusedly in the greenish gloom. Brought from all the recesses of the coast in all the legality of time contracts, lost in uncongenial surroundings, fed on unfamiliar food, they sickened, became inefficient, and were then allowed to crawl away and rest.
Conrad immediately compares this scene with the figure of the Company accountant who is, clean, immaculately attired, and keeps his books in "apple pie order." He complains when the natives speak or move: "When one has got to make correct entries," he says, "one comes to hate those savages—hate them to the death."
And another layer of the onion peels away: Kurtz is the epitome of the true heart of darkness, of pure evil. He has lived among the natives for so long that he's "gone native," having long since lost what thin veneer of civilization he had. He has used his influence and superior firepower and fear to become godlike to the natives--Marlow refers to him as a "pitiful Jupiter," as he is sick when they meet--and his home is surrounded by heads on stakes, "black, dried, sunken, with closed eyelids...,that seemed to sleep at the top of that pole, and, with the shrunken dry lips showing a narrow white line of the teeth." When Kurtz is taken onto the steamboat, near death, Marlow wonders "how many powers of darkness claimed him for their own."
And finally, the heart of darkness lies within each of us. Note that "all Europe contributed to the making of Kurtz." Marlow tries to explain:
You can't understand. How could you?—with solid pavement under your feet, surrounded by kind neighbors ready to cheer you or to fall on you, stepping delicately between the butcher and the policeman, in the holy terror of scandal and gallows and lunatic asylums—how can you imagine what particular region of the first ages a man's untrammeled feet may take him into by the way of solitude—utter solitude without a policeman—by the way of silence, utter silence, where no warning voice of a kind neighbor can be heard whispering of public opinion? These little things make all the great difference. When they are gone you must fall back upon your own innate strength, upon your own capacity for faithfulness.
In other words, we are all savages just under the surface. It is only the superficial laws and customs of our existence that divide us from becoming Kurtzes (the next time you hear of soldiers at war committing "atrocities," remember this). Marlow tries to explain this, but finds that words still fall short of expressing this critical idea.
In the treatise Kurtz had penned for the International Society for the Suppression of Savage Customs, Kurts had first written (with breathtaking eloquence) that whites "appear to them [savages] in the nature of supernatural beings—we approach them with the might as of a deity,' and so on, and so on. 'By the simple exercise of our will we can exert a power for good practically unbounded...." Only, at the end, written much later, were the unsteadily scrawled words: "Exterminate all the brutes!"
Presumably, Kurtz had decided that all the natives were brutes and could not be saved, but the reader is left to decide who, precisely, the brutes really are.
Kurtz's famous last words, aboard Marlow's steamer, were "The horror! The horror!" We are left, again, to determine what the most horrible behavior in this book is.